Is There a Magic Number for Democrats to Take Back the House?

As the presidential campaign enters its final stretch, many people have begun to ask me: "What size margin would Hillary Clinton need to win the White House by in order for Democrats to win back the House?"

To be honest, there may be no such magic number. Conventional wisdom holds that a landslide Donald Trump loss could imperil Republicans down the ballot, because straight-ticket voting has been on the rise in recent years. But thanks to Trump, 2016 is not at all a "normal" year, and the relationship between the top of the ticket and the down-ballot may not be so linear.

It's no secret that both candidates' unpopularity has created a large bloc of "nose-holding" voters who see their choice as the lesser of two evils. And a larger Trump loss wouldn't lead to a mandate for Clinton and Democrats, who need to flip 30 Republican seats to win control of the House of Representatives. In fact, the more apparent it is that Trump will lose, the more voters could be enticed to keep their local Republican in Congress as a "check" on Clinton.

Instead, a much more useful down-ballot indicator could be the "generic congressional ballot" poll question: a variation of the question, "If November's election for Congress were held today, which party's candidate are you more likely to vote for in your district?"

An early August McClatchy/Marist poll found that 49 percent of registered voters favored Democrats, while 41 percent favored Republicans. An NBC/Wall Street Journal poll released around the same time found registered voters preferred Democrats to control Congress, 47 percent to 43 percent. RealClearPolitics and HuffPost Pollster averages, which include many more online surveys, show similar leads for Democrats in the 4 percent to 5 percent range.

So how would that polling lead translate into actual House seats?

The truth is, it's a bit murky. At first, you'd think these polling numbers would prompt Democrats to stock up on champagne. After all, the last time House Democrats took back the majority in 2006, they won 52 percent of all votes cast for House to 44 percent for Republicans -- an 8-point advantage -- and reclaimed the chamber by a healthy margin, 233 seats to 202 seats. However, the House landscape in 2016 looks a lot different from 2006.

To simulate the impact of shifting national winds on the House majority, let's start out with the district-by-district results of the last election as a baseline.

In the 2014 midterms, Republicans carried about 51.4 percent of all votes cast for House to 45.7 for Democrats, and wound up with a 247 to 188 seat majority. Let's say that in 2016, every Democratic candidate performs 6 percentage points better than their party's 2014 nominee, and every Republican performs 6 percentage points worse. Democrats would win the House popular vote by 6.3 percentage points -- roughly in line with their current generic ballot lead.

But amazingly, even in this "uniform swing" scenario, Republicans would still hold the House majority by nine seats. Here are the only 21 seats Democrats would gain:

What's worse for Democrats, this 21-seat gain is an overly rosy outcome given the political realities in each of these districts. There are five congressional districts -- Arkansas' 2nd and 4th, Georgia's 12th, New Jersey's 3rd and West Virginia's 2nd and 3rd -- that Democrats strenuously contested in 2014 but aren't seriously contesting in 2016. So in reality, even if Democrats won the national vote by 6 points, they could still only pick up 15 seats -- about half of what they need.

Now, let's get really aggressive and test what would happen if every Democrat were to outperform their party's 2014 nominee by 7 points and every Republican were to underperform by the same. Democrats would win the national vote by 8 points, the same margin as in 2006. But they would only flip six additional GOP seats, getting them to a gain of 27. Here are the additional seats they would gain:

Even throwing in Democrats' virtually guaranteed gain of two seats from new court-ordered congressional maps in Florida and Virginia, they would still fall one seat short of the majority. And once again, these additional gains are likely wishful thinking for Democrats: they aren't seriously contesting Michigan's 8th Congressional Distct (where Little House on the Prairie actor Melissa Gilbert dropped out in May) or New York's 11th Congressional District (where they had the luxury of running against an indicted incumbent in 2014).

So why the disconnect between the House popular vote and the seat count? If an 8-point lead in 2006 got Democrats the majority then, why wouldn't it now? Even more dramatic than Republicans' emerging disadvantage in the Electoral College, Democrats are at a serious structural disadvantage in the House for three reasons:

Republicans redrew congressional lines in their favor in 2012 redistricting: In the 2010 midterms, Republicans picked up over 600 state legislative seats, allowing them to "pack" Democratic voters into urban seats in state after state following the Census. This has had the effect of making the House much less elastic. In 2012, Republicans won 33 more House seats than Democrats despite winning one percent fewer votes.

Democrats start out with far fewer personally popular incumbents than in 2006: Ten years ago, heading into the 2006 election, there were 41 Democrats sitting in seats that George W. Bush had carried in 2004. A majority of them were conservative Blue Dog Democrats from the South, including the likes of Gene Taylor in Mississippi, John Spratt in South Carolina, and John Tanner in Tennessee. All 41 of those seats stayed in Democratic hands in 2006.Today, thanks to retirements and defeats, only 5 of those 41 Democrats remain in Congress, and the lost seats in places like Alabama and West Virginia just aren't coming back. In fact, there are only five House Democrats sitting in seats that Mitt Romney carried in 2012, and only two of them are running for reelection this year. That means Democrats start out in a much deeper hole than they did ten years ago.

Democrats haven't expanded the House playing field beyond their top targets: Back in 2006, Rahm Emanuel was the chair of Democrats' House campaign committee and employed notoriously aggressive techniques to recruit top candidates to run. This year, it wasn't apparent Trump would lead the GOP ticket until May, and by that time, filing deadlines had passed in 81 percent of districts. In some cases, Democrats are leaving seats on the table.

The bottom line is: House Democrats will gain seats in 2016, but there probably isn't any size Clinton victory that could guarantee them the majority. The question may be less how much Republicans will overperform Trump and more to what extent Trump underperforms other, more traditional Republicans. In the end, there's a good possibility House Democrats will win far more votes than Republicans but still win fewer House seats, much as they did four years ago.